Making a PCB using the UV light method

This is a step towards PCB prototyping. Some time ago a similar post was published but it concerned making a PCB using a thermo transfer method. If you need to prepare a PCB fast this method is probably the best choice.

What is different about the UV light method? Why is it worth your attention. The answer is simple — quality. Then how to make it? Keep reading …

Brief description of the process

To make a PCB using UV light you need a few things. First, you need to prepare a piece of FR4 laminate; it should be clean and degreased. Second, a layer of photosensitive film on a piece of laminate. Third, a template of mosaic of electrical connections. The mosaic should be printed in negative. Forth, a source of UV light. During irradiation the photosensitive layer which is exposed to the UV light hardens. After that you need to develop the film and wash off the parts which were not exposed to UV light. This way, on a PCB, only electrical connections are left. Now the laminate can be etched, during this process only parts which are coated with hardened photosensitive layer will resist to the etching mixture. Now some cleaning. Lo! You have a PCB.

General remarks

While using thermo transfer method it is not possible to achieve track width of less than 16 mils. Sometimes it is doable but it requires a lot of practise. For this you can use lamination device and it is possible to achieve as little as 10 mils. However, 16 mils is more than enough for the majority of projects.

There is also another condition that has to be meet. Good quality of printout is highly demanded.

In case of the UV light method toner is also used. However, it is not transferred to the copper layer. Instead the copper layer, with two other things, is exposed to the UV light.

This post is intended to address the process of making a PCB using the UV light method along with some remarks which hopefully are useful.

The method is fairly simple but has a one or two more steps than the thermo transfer method. However, there are two (three) ways of employing the UV light method. It depends how the photosensitive layer is deposed on copper. For this purpose a Positiv 20, a can of photosensitive liquid, can be used. It is a bit cumbersome because of the conditions in which the copper has to be sprayed. First of all, the copper has to be clean, it is the common condition for all methods, and during dusting with Positive 20 it is crucial that no dirt stick to the board. After that the PCB has to be baked in the oven or lay in room temperature for 24 hours to get dry. It is worthy of note that this has to be done with no sunlight, at least direct sunlight. If this seems to complicated it is possible to buy a FR4 laminate which is pre-coated.

However, the Positiv 20 method will not be described here. Instead, a UV method that makes use of photosensitive film will be described in detail. Using photosensitive film is very good for beginners because it is extremely forgiving, however, to a certain point.

Laminate preparation

First step is to adjust the size of the FR4 sheet. It can be done with hacksaw blade.

A piece of laminate has to be wet sanded. For this 1000-grid sandpaper is perfect.

Degreasing

The best way to degrease is to use washing powder.

The surface after proper degreasing should be adhesive for water. In other words, after application of some water droplets on the copper surface the liquid should spread across it.

The tactile sensations during the contact with degreased surface can be misleading. The slippery sensation, however, is to be expected.

Film application

It is important to prepare the room in which the application will be performed. It should be shielded from direct sunlight, although it does not have to be entirely dark. Using red light can improve visibility. Good practice it to draw curtains in the room.

The photosensitive film is transparent with a blue tint. It is transported in a tube and the film itself is wrapped with black foil.

In full sunlight after several seconds the light blue turns into dark blue or even dark purple shade. After sunlight exposure the foil becomes useless.

Below a sheet of partially exposed film to direct sunlight.

The sheet consists of 3 layers. Two of those are protective layers and the inner layer is photosensitive.

Two protective layers are seemingly identical. If the film is transported in a tube then the inner layer is flexible and much more extensible while the external layer is harder. Before doing anything with the film it is crucial to identify which layer is which.

After successful identification of layers the film can be applied on the copper. Put the FR4 laminate on a flat surface. After that remove the flexible protective layer from the film. Put the film on the piece of laminate, apply the film facing down. It has to stick to the copper. To ensure good cleaving use a soft roll and press the film to the piece of laminate firmly but carefully. This is to ensure that all air bubbles will be eradicated. While pressing the film keep in mind that no heat source should be used.

Next step is to prepare an iron. Unfortunately, here, analogies to the thermo transfer method are over. The temperature knob on the iron should be set to the lowest.

Warning!

Basically, if you briefly touch the iron with the back of a hand it should not be hot! Even with longer contact it should be bearable! Keep in mind that the iron can burn the skin if used improperly. Always a single drop of water can be used to verify if the iron’s temperature is not set to high. Never the less, be very cautious!

Iron’s foot should be clean and dry. The iron should be positioned facing up. On the iron’s foot a clean sheet of paper should be placed and on top of it the PCB with film facing up. The extra sheet of paper is important so the film will not stick to the iron. The FR4 laminate should be then pressed with soft-material roll to ensure good stickiness of film to the PCB.

After the set cools down gently tear off the PCB form paper. It is only required if the film was larger than the piece of laminate which is recommended. Do not pull the film, instead pull only the sheet of paper and PCB. You can touch the film now since it is protected by additional layer that was mentioned earlier. The excess of the film will stick to the sheet of paper but the protective layer should still be attached to the PCB. Do not tear off the protective layer of the film.

In case of a two layer PCB the hole process should be repeated. Keep in mind to use a clean sheet of paper because the excess of the film from the first layer can stick to the film.

Template preparation

The mosaic of tracks is printed on a transparent sheet of foil. The print out should be a negative one. In contrast to thermo transfer method the track should be transparent.

In case of 2-layer PCBs two templates should be printed out. The first layer should be printed mirrored and the bottom layer without any alterations. A number of pads but not as many vias is recommended. It will make positioning of the layer much easier. If there is not enough number of above it is recommended to add some points/mounting holes on the edges in CAD software.

The foil should be trimmed and aligned.

It is good to leave some foil on two sides as showed above. After trimming the templates, they should be aligned. To make it easier place templates on a white sheet of paper or look up at the sky through the templates. To fix the alignment of the two parts use some duck tape. Remember to apply the duck tape on the shorter edges. It will be easier to place a piece of laminate afterwards. Also, keep in mind that the toner layer of template should be facing towards the PCB rather than from it.

UV irradiation

Place the PCB between the prepared templates and fix it.

Now the set needs to be exposed to the UV light. As UV light source a UV nail lamp can be used. It can be bought for less than 20 USD. In case of this post a UV nail lamp with 4 ultraviolet lamps of 9W each was used.

The process of irradiation is as follows. Place the set inside the UV nail lamp. The template with mosaic has to be pressed to the piece of laminate. A sheet of glass can be used, i.e. the thickness of the glass should be 2 to 4 mm. The glass sheet can be weight on with small pieces of lead.

The exposure time differs. It mainly depends on the power of UV lamps and the distance between source of UV light and the PCB. In case of this set-up the optimal time was 70 seconds.

Warning!

Do not look directly at the source of UV light!

The time of exposure should be adjusted. A good determiner for the time of irradiation is the color of the film. Around 30 seconds after the color of the film will become dark blue it can be assumed that the photosensitive emulsion was hardened enough. Also the time of 30 seconds is dependant on a few factors described above. Too short or too long UV exposure is not good for further stage — development of the film. Therefore, the time should be just right! Using below ratio can also make the exposure time easier to determine.

4:3

4 is the period required for the film to turn dark blue and 3 is time to make sure that the film hardened enough. For example, the film turned dark blue after 20 seconds so the total time of irradiation is equal to 35 seconds.

The PCB after UV light exposure was presented below

After irradiation the template should be gently separated from the PCB. The mosaic template can be successfully reused.

The PCB should be left in a dark container to lay for around 15 minutes. Before that it is important to examine how the process of exposure was carried out. Extremely important is the sharpness of track edges. If the tracks are even a bit blurry the laminate should be cleaned and the whole process described to this point should be repeated. The reason for smudged edges could be following:

the glass might not equally press the template to the copper,

the process of the irradiation could have been exposed to vibrations.

Developing film

This process is very similar to the process of developing photos. However, in the context of making PCBs the developing of the film is about to make the photosensitive substance insensitive to the UV light. Before starting to develop the PCB the external protective layer of the UV foil should be removed. It should be done very gently because the photosensitive layer is now very fragile.

Furthermore, the process requires different set of chemicals adequate to the task — sodium metasilicate.

Warning!

The sodium metasilicate is dangerous substance proceed with caution!

During the process of development the areas exposed to the UV light that turned dark blue are hardened. Moreover, the areas which were shielded from light are degrading and become liquefied.

The ratio for the mixture should be chosen accordingly to the information placed on the package. During the development it is advised to gently swipe the PCB with soft-bristles brush. After that wash the PCB under running water.

Etching

The technique described here was inspired by an article from EdW (Elektronika dla Wszystkich, a Polish magazine).

In this process B327 is used — sodium persulfate. It eats out the uncovered areas of copper.

A small zip bag is used to contain the etcher and the PCB. This way the amount of B327 is not as high as etching PCB in a container. Add right amount of sodium persulfate and around 100 ml of warm running water. Gently shake the bag so the B327 melts. After that put inside the PCB. Do not let air out of the bag completely. If the bag is to big just roll one of the edges and fix it wit a piece of duck tape.

During the process it is advised to shake the bag gently. The air bubble should not be left in one place because it can disturb the process. The etching is very rapid. From time to time put the bag into a container with warm water. This rises the temperature of the solution inside the bag.

Etching PCB in a zip bag allows to save resource in form of the B327, thus the use of sodium persulfate is more effective. At the end of etching the solution does not need to be stored for reuse. This is very economical way of using sodium persulfate.

Cleaning up!

There is a very simple method which allows to remove photosensitive emulsion, and this is not a sandpaper. Using sandpaper is cumbersome and still there can be some missed spots. The best way is to use abstergent for cleaning pipes. The main ingredient of that kind of detergents is sodium hydroxide and aluminium pellets.

Warning!

Sodium hydroxide is very dangerous! Cleaning with sodium hydroxide should be performed in well-ventilated rooms. Always use protective glasses and mask. Sodium hydroxide can lead to upper airways irritation. Proceed with caution!

To a container poor some water and add one or two teaspoons of the abstergent. Put PCB inside. After just a few seconds dark blue strands can be observed.

Cleaned PCB was depicted in photos below.

Soldermask

Applying soldermask does not always has to be problematic. The first step is to prepare appropriate templates. In a CAD software select only pads and vias to be visible and print in negative. Apply a new sheet of photosensitive film. Use prepared templates to irradiate the PCB and after that develop it. During the development stage the unexposed photosensitive film will be removed.

Drilling and vias

After applying soldermask there is a time for drilling and making vias. To make a via just drill the spot were a via is supposed to be. Inside the drilled hole place a wire. Solder it from one side. It should not stick out. After that solder the other end and cut the wire.

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